Synthesis of ZnO nanoparticles and its characterization

Abstract

Nanomaterials (NMs) tailored via conventional physicochemical routes play havoc with the environment which has led to the evolution of competent green routes for the actualization of a circular economy on an industrial scale. “Green route” of synthesis of nanoparticles has emerged as a revolutionary approach. The current study aimed to exploit plant and algal-mediated synthesis of ZnO nanoparticles (ZnO NPs), optimize the process for gram scale synthesis and use the ZnO NPs for the photocatalytic degradation of textile dyes (brilliant green and indigo caramine) and organic pollutants (bisphenol-A). One-step biosynthesis of ZnO NPs from biomolecules in lemon grass extract was carried out. The main objective was to investigate the role of aqueous extract of lemon grass in the production of ZnO nanoparticles. Different molecules were present in the crude extract of Lemon grass were identified by GC-MS and NMR (13C and 1H NMR) spectroscopy and observed 23 bioactive compounds in the extract rich in various forms of terpenoids, monoterpenes, keto-enol compounds, fatty acids, palmitic acid, and phytol along with some other ancillary phytochemicals which could be potential candidates for capping agents. The abundant presence of citral and photocitral-B in lemon grass extract acted as a coating and provides stability to ZnO NPs. Eco-friendly lemon grass capped ZnO NPs were synthesized using lemon grass extract and confirmed using different characterization techniques like UV- Vis spectroscopy, XRD, SEM and HR-TEM analysis. We found the size of the lemon grass capped ZnO NPs as 43.58± 3.2 nm and the shape of the particles as hexagonal through the HR-TEM analysis Phycosynthesis of ZnO NPs using a single-step process, from biomolecules present in the cyanobacterial extracts of Anabaena variabilis ARM 441 was carried out and analyzed for photocatalytic degradation of textile dyes. The primary focus was to explore the role of aqueous cellular extract of diazotrophic cyanobacterium A. variabilis ARM 441 in the fabrication of ZnO NPs. Bioactive components of algal extracts identified by GC-MS and NMR (13C and 1H NMR) spectroscopy, reveals 21 different compounds, among which n-hexadecanoic acid and 13 tetradecenal had properties of reducing and capping agent required in the synthesis of ZnO NPs. Microscopic investigation of particle size and zeta potential confirmed the formation of hexagonal ZnO NPs with an average size of 33.31 nm. The EDX and XPS analyses established ix the chemical composition and high purity of ZnO NPs. The rietveld refinement studies of X-ray diffraction studies elucidated crystalline and wurtzite phase of ZnO NPs. Pore size (11.551 nm), surface area (38.718 m²/g) and pore volume (0.1633 cc/g) were studied by BET analysis. In order to optimize the synthesis process response surface methodology (RSM) was applied. To get gram scale yield, BBD (Box-Behnken Design) at different parameters was employed to both lemon grass and A.variabilis ARM 441 mediated synthesis of ZnO NPs. Impact of three crucial paramters, i.e. zinc concentration (mM), reaction time (h) and extract concentration (%) was investigated in both the cases. In case of gram scale synthesis for lemon grass mediated ZnO NPs, numerical optimization result revealed that the hydrothermally green synthesis of ZnO NPs using 550 mM zinc salt concentration, 45% lemon grass extract for 8 h could yield 1857.2 mg ZnO NPs. Similarly, in case of A.variabilis ARM 441 capped ZnO NPs in co-precipitation method, the production was most affected by extract concentration with the highest F-value (369.96). Zinc ion concentration had the second-highest F-value (244.78), whereas reaction time had the lowest (9.69). The overall nanoparticle yield of 1565.26 mg was reported at optimized conditions of 250 mM (Zn2+), 30% algal extract for 6 h. The nanoparticle yield was dependent on the extract's reducing ability (%), which affects zinc ion availability. The rapid interaction between zinc ions and bioactive compounds in the extract reduced the reaction time. Further, the photocatalytic behavior of Lemon grass (Cymbopogon citrates) synthesized ZnO NPs was estimated by the photodegradation of Bisphenol-A (BPA) under UV illumination. After 45 min of exposure of BPA to UV light, about 97.41 % of BPA was degraded with a rate constant 5.15 × 10−2 min-1. The photodegradation of BPA followed pseudo first-order kinetics with catalytic dosage (50mg/100mL), BPA concentration (0.5 mM); pH (6). Further, in the reusability experiments, it was observed that after 5 cycles, the degradation efficiency of ZnO NPs for BPA was 86.9%. Photodegradation was also observed, and its degradation products were analyzed by LC-MS analysis and a metabolic pathway was inferred. The photocatalytic behaviour of A.variabilis ARM 441 synthesized ZnO NPs was estimated by the photodegradation of Brilliant green (BG) and Indigo caramine (IC) under UV illumination. A comparative study was performed to test the photocatalytic efficiency of ZnO NPs with cationic x and anionic dyes. The photodegradation of BG dye followed first-order kinetics with catalytic dosage (50 mg/100 mL), dye concentration (30 mM); pH 7.0. However, in the case of IC, after the exposure of 130 minutes, about 80.8% of the dye was degraded, and it also followed firstorder kinetics with catalytic dosage (50 mg/100 ml), dye concentration (30 mM, and pH 5.0. The decolouration reusability experiments for BG and IC were performed. It was observed that after 5 cycles, the degradation efficiency of ZnO NPs for BG and IC was 88% and 69%, respectively. The degradation products were studied via LC-MS analysis and identified by interpretation of their m/z value. Various by-products of lower masses (m/z) at different degradation stages were observed. The opening of the aromatic rings occurred and aliphatic compounds were converted into oxalic acid, which ultimately oxidized into CO2 and H2O. Based on the produced metabolites, a degradation pathway was proposed. The ability of as-prepared ZnO NPs to produce an extensive amount of reactive radicals (such as superoxides, hydroxides and peroxides) owing to their large surface area makes it an eligible photocatalyst. Biosynthesized ZnO NPs showed evidence to act as an effective photocatalyst, which is appropriate for industrial wastewater treatment, especially to degrade harmful and toxic pollutants that persist in aquatic environment.